Applying armature makes the mesh deform in very weird ways.

[QbieShay]

If the model is exported when it's not in base pose, the armature modifier gets applied to the model, leading to the model to be deformed in a not proper way. There should be a way, when exporting the model, to exclude the armature modifier from being applied to the geometry. Even in base pose, the armature modifier should not be applied because, depending on the type of rig, even the armature in complete rest position could differ from the original model due to bone constraints.